Chronic hepatitis C virus infection has emerged as the most important form of viral hepatitis in the United States. Although the disease often remains indolent for many years, approximately 20 percent of patients with chronic hepatitis C progress to cirrhosis within 20 years. As a consequence, liver failure from chronic hepatitis C has become the leading indication for liver transplantation both in the United States and Europe. Despite remarkable progress in our understanding of many aspects of this disease, the mechanism of hepatocellular injury in patients with chronic hepatitis C is not well understood. Unfortunately, neither efficient cell culture systems nor animal models are available to study the pathogenesis of the liver disease. Given these limitations, we have systematically examined the evolution of liver injury in patients with recurrent hepatitis C after liver transplantation. By correlating the degree of injury to the transplanted organ with detailed analyses of viral replication we hope to provide insights into the pathogenesis of liver cell injury in these chronically infected patients. This proposal is designed to examine the hypothesis that differing patterns of mutation in the genomic sequence of HCV RNA have a profound influence on the severity of liver injury in patients with recurrent hepatitis C after liver transplantation. Our Specific Aim 1 is to prospectively determine the relationship between evolution of HCV quasispecies and hepatocellular injury in patients with recurrent hepatitis C. In Specific Aim 2 we plan to intensively investigate the clonal evolution of quasispecies and to determine the tissue origin of these viral strains both in patients with severe recurrent hepatitis C and in those with mild hepatocellular injury after liver transplantation. We plan to test the hypothesis that the primary site of quasispecies replication is critical to the development of hepatocellular injury. We propose that emergence of nonpathogenic strains of virus after liver transplantation result from continuous extrahepatic replication of HCV, whereas pathogenic strains of HCV originate from intrahepatic replication of the virus. In Specific Aim 3 we plan to assess in impact of HCV genotype, circulating levels of virus, and evolution of HCV quasispecies on the long term evolution of histologic injury in the transplanted liver. By carefully correlating the evolution of the virus with the degree of hepatocellular injury experienced by patients who develop recurrent hepatitis C after liver transplantation, we hope to provide insights into to the pathogenesis of this important liver disease. From these detailed analyses we also hope to isolate pathogenic strains of virus which can later be tested in cell culture systems.